Among materials which change in color when exposed to light is a photochromic substance. The photochromic substance has the property that its structure reversibly changes according to the presence or absence of ultraviolet radiation to change in absorption spectrum. This property results from the nature that when an isomer is irradiated with light of a particular wavelength, the single chemical substance, under the action of light, reversibly produces an isomer having a different absorption spectrum. The resulting different isomer is restored to the color of the original isomer by heat or light of a different wavelength.
Photochromic spectacles utilize the above nature of the photochromic material for lenses. In an outdoor environment exposed to light including ultraviolet radiation, such as sunlight, the photochromic spectacles have their lenses rapidly colored to function as sunglasses. In an indoor environment without exposure to light, the lenses fade to function as ordinary transparent spectacles.
Known methods for producing a lens having photochromic property include a method of impregnating the surface of a non-photochromic lens with a photochromic compound; a method dissolving a photochromic coating fluid in a monomer, and polymerizing the solution to obtain a photochromic lens directly; and a method providing a layer having photochromism on the surface of a lens with the use of a coating agent containing a photochromic compound.
Of the above-described methods for producing a lens, the coating method, which coats a photochromic coating fluid onto a lens, involves ejecting a photochromic coating fluid from the nozzle of a barrel containing the photochromic fluid to the surface of the lens, while rotating the lens, to carry out spin coating, thereby forming a coating layer on the surface of the lens. For example, the technology of Japanese Unexamined Patent Publication No. 2000-334369 is available as an apparatus which can coat a plurality of lenses continuously, and the technology of Japanese Unexamined Patent Publication No. 2005-013873 is available as an apparatus equipped with an auxiliary mechanism for spreading a coating fluid on a lens with the use of a flexible film.
In spin coating with the photochromic coating fluid, it is not necessary to change the setting of the apparatus, if the lens of the same type and the same size and having the same gradient is to be coated. However, if the coating fluid is to be applied to the lens different in size and gradient, it is necessary to adapt the apparatus to the shape of the lens. In this case, if the height and gradient of the lens are known beforehand, the setting of the apparatus can be adapted to their pattern. If the height and gradient of the lens are unknown, it is common practice to investigate the height and gradient of the lens anew, and then adjust the height of the lens or the height of the nozzle in accordance with a position suited to the lens. In spin coating the lens with the photochromic coating fluid having high viscosity, moreover, the problem arises that even in the presence of centrifugal force during rotation, the coating fluid builds up at a peripheral edge portion of the lens, and drops onto the side surface of the lens.
The present invention has been accomplished in the light of such circumstances. It is an object of the invention to provide a coating apparatus which can detect the height and gradient of a lens automatically and easily, and can form a stable photochromic coating film.